Slide an object toward a lens. Follow the rays, find a sharp image, and discover the moment a camera becomes a magnifier.
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A lens changes the directions of light rays. A real image forms where outgoing rays meet. A virtual image appears where their backward extensions meet—even though light does not actually go there.
The object moves inside a converging lens’s focal length. Can a screen behind the lens catch its image?
The rays leaving the lens spread apart. Their backward extensions locate the virtual image on the object side. Try the magnifier preset and inspect the dashed lines.
Parallel rays entering a converging lens meet at its far focal point in the thin-lens approximation. A diverging lens spreads those rays as if they came from its near focal point. The lens shape is a symbol for that behavior, not a glass design.
To record a sharp image, a camera places a sensor where a bundle from each object point converges. Move the virtual sensor through that position and compare the bundle width. Real cameras usually focus by moving lens groups; this bench moves the sensor to expose the geometry.
Place an object inside a converging lens’s focal length. The outgoing rays diverge, but their backward extensions locate an upright virtual image. A screen behind the lens cannot catch that image. Your eye can receive the rays and form its own retinal image.
1/f = 1/do + 1/di. A real object has do > 0. A converging lens has f > 0; a diverging lens has f < 0. A positive di means a real image on the far side. Negative di means a virtual image on the object side.
Transverse magnification m = −di/do. Negative m is inverted. At do = f for a converging lens, the emerging bundle is parallel: there is no finite image distance. The calculator reports this explicitly rather than dividing by zero.
For an on-axis point and aperture radius a, the geometric bundle radius at sensor distance s is a|1 − s/di|. The camera inset uses this ray geometry. Diffraction, aberrations, wavelength dependence and real sensor sampling are omitted.
Indoors, hold the magnifier just above the printed text. Look through it at the letters. Keep the paper and your viewing position reasonably steady.
Move the lens slowly away from the page while looking through it. Notice whether the letters look upright, larger, blurred or eventually inverted. Stop whenever viewing is uncomfortable.
Sketch one position that gave enlarged upright letters. Compare it with the magnifier preset. Do not estimate the lens’s focal length from a single viewing distance: your eye and lens positions both matter.
How does the text change as the lens moves a little farther from the paper?
Use indoor light only. Never look at the Sun or direct sunlight through a lens, and keep magnifiers out of sunlight. The activity is a qualitative observation.
University Physics Volume 3 §2.4, Eqs. 2.19 and 2.22, sign conventions and ray rules. Supports real, virtual and infinite-focus cases.
OpenStax · Thin lensesUniversity Physics Volume 3 §2.6, optical focusing and detector discussion. We use the general optical principle, not its dated claims about particular smartphone mechanisms.
OpenStax · The cameraUniversity Physics Volume 3 §2.8, compound microscope explanation. Supports the simple magnifier arrangement; no microscope model is implemented.
OpenStax · Optical instrumentsIndependent subject review is pending.
Read the sources and model assumptions
Interactive explorationStart with a vibration. Follow air, eardrum, tiny bones and hair cells, then see why the message reaching your brain is a different kind of signal.
Close a switch. Follow a complete path. Give current a second route and discover what changes.
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